As a conventional method of optically recording and reproducing consecutive signals such as video signals or audio signals on an optical disk, it is known to record information on the disk by making a mechanical convexo-concave pattern and to reproduce the recorded information be detecting the convexo-concave pattern using the interference of reflected light from the recesses and the convex portions. In order to produce a master disk in accordance with the above mentioned conventional method, a substrate disk is coated with a photoresist layer, and then a laser beam which is focussed to be a narrow beam, and which is modulated with an input recording signal is incident on the disk. After this process, the regions of the photoresist which have been irradiated with a laser beam are removed by etching to form a convexo-concave pattern on the disk. The above mentioned conventional method of recording and reproducing information is not practical for home use since it includes complicated steps in recording. Especially, the necessity of the etching process constitutes an obstacle in the application of the method into home use. Furthermore, such a disk cannot be covered by a suitable protective film or layer prior to recording since deformation has to be made to perform recording. Accordingly, it is difficult to handle such an uncovered disk at home.
According to another conventional method of recording information on a disk by a laser beam, with a substrate disk coated with a metal, such as bismuth or a semimetal by vaporization, given portions of the metallic film are melted or holes are made in the metallic film by evaporating the same by the heat applied by an Ar laser incident ray. This method also has a drawback that a protective surface layer cannot be provided since the film made by vapour coating has to be deformed, while it is required to use a laser the output power of which is relatively high.
Recently, amorphous chalcogenides are given attention as an optical recording material. Although holograph recording by means of an Ar laser beam on amorphous chalcogenides has been attempted, the application of the amorphous chalcogenides to video disks using a semiconductor laser has not yet been introduced.
A remarkable progress has been made recently, in connection with a semiconductor laser and as the result of the progress, it is now possible to manufacture a semiconductor laser the life time of which is over 1,000 hours when used under the condition of continuous oscillation with the output power between 10 and 40 milliwatts. However, even though such an improved semiconductor laser is used, it has been still difficult to record information, such as video signals, on an optical disk immediately inasmuch as a suitable structure of an optical disk which matches the characteristics of consecutive wave type semiconductor laser has not yet been developed.